Quartz may be chosen as the material for HID arc tubes because quartz can withstand high pressure of about 200-300 times of the atmospheric pressure; on the other hand, arc tubes made of quartz is easy to be encapsulated or sealed. However, quartz materials also have apparent disadvantages; i.e. on one hand, the dimension of the inner chamber of the quartz arc tube has uncontrollable tolerance, since the quartz arc tube is formed by molding hot melt at a high temperature by pressing outer mold against the inner chamber while blowing highly pressurized gas into the inner chamber; on the other hand, when the quartz arc tube operates at a high temperature, the color rendering property deteriorates with reduced luminous efficacy and stability, and the lost of sodium in the lamp is severe.
Quartz arc tubes, especially low power lamps used for exhibition lighting, are replaced by polycrystalline alumina (“PCA”) ceramic arc tube to significantly enhance the consistency and stability of light color. The replacement of quartz arc tube by ceramic arc tube improves the color consistency of metal halide lamp (“MH”), one reason is that the base body used to make ceramic arc tube is shaped by molding or grout molding at an ordinary temperature, so that its dimension is well controlled; and another reason is that ceramic arc tube increases the operation temperature of the tube wall. Thus, in case that the temperature of the tube wall is high, a minimal value can be observed in the curve of correlated color temperatures versus temperatures of the tube wall. Ceramic arc tube is adapted to operate in a region around the corresponding color temperature minimal value (by contrast, for quartz arc tube, such minimal value area is above the appropriate endurable operation temperature of quartz glass), especially when the filling agent is halides of sodium and rare earth element, a higher operation temperature enables the lamp to have a better color rendering property (Ra>80) and higher luminous efficacy (>90 lm·W−1). Another benefit of the structure of polycrystalline alumina is in its lifetime, the loss of sodium within the lamp is greatly reduced, ensuring that the color rendering property is more stable than conventional metal halide lamp using quartz arc tube.
Chinese Patent No. 98115658.4, the entire disclosure of which is incorporated by reference herein, discloses a ceramic shell component of high intensity discharge lamp, which has a shell structure of PCA shell, and specially designed multi-layer structure of axially delaminated aluminum oxide-metal ceramic. The terminal therein provided with multiple elements has its last element directly sintered to the corresponding feed-through wire without any sealing material, while keeping a certain thermal expansion coefficient. However, in fact, due to high operation temperature and intense corrosion due to fused metal halide and steam of the lamp, such sealing methods cannot provide long-term reliable sealing and a longer lifetime.
U.S. Pat. No. 6,313,582, the entire disclosure of which is incorporated by reference herein, discloses a ceramic lamp, in which the discharge tube is made of translucent ceramic, and the tube sealing part is terminally encapsulated by means of Dy2O3—Al2O3—SiO2 based sealing material. It may be difficult to encapsulate the electrode of such arc tube made of ceramic due to poor pressure resistance of PCA itself (since it may only withstand about 3-5 times of the atmospheric pressure). In the actual state of air exhaust, it may not be easy to fill metal halide balls, inject mercury, inflate various inert gases into the arc, and maintain gas sealing between the outer lead of electrode and ceramic material by filling glass solders. The last step of manufacturing arc tube may fail, meaning that all the previous steps were wasted, and the yield rate of the arc tube is affected, and the costs cannot be lowered. Moreover, the technique in the art may only enable an arc tube with power of 150 W or lower, as an arc tube with high power of 250 W or up to several KW is not commercially applicable.